Seamanship 3 - Final Module 1) [PDF]

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FINAL MODULE 1 Explain, present the process, chemical reaction identification of Corrosion onboard modern day vessel.



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Deterioration of ships hull / structure through corrosion, fatigue and damage is identified as a principal factor in the loss of many ships carrying cargo in bulk . Failing to identify such deterioration may lead to sudden and unexpected accident. Bulk carrier crews may be unaware of the vulnerability of these vessel types. The consequential loss of a ship carrying heavy cargo can be expected to be very rapid, should a major failure occur. The



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Ships Corrosion Ships are built of steel, which in a marine environment exposed to water (both fresh and sea) and air is prone to the formation of rust. Contributing factors that accelerate the rate of corrosion include: 1. Cargo damage this occurs when heavy bulk cargo is allowed to freefall from height onto the tank tops. The heavy impact of this cargo on the tank top causes damage and breakdown of the coatings on the ceiling of the double bottom tank underneath 2. Corrosive cargoes a number of bulk cargoes contain chemicals of a corrosive nature and this is particularly the case in newly mined coal. It is essential that the data sheet is inspected prior to loading the cargo. For example, in the case of a high sulphur contact coal cargo, severe pitting can result. To counter this, the hold floor can be coated in lime, but this does not protect the bilges or bilge lines 3. Equipment damage grab damage to the hold floor, frames and ladders can occur at most discharge ports. This not only causes material damage to the ship's structure, but can also break down the paint coatings exposing the base steel to the atmosphere. The deliberate hammering of the floor and sides of the hold by grabs and bulldozers to free cargo residues trapped between the frames will result in structural damage and the breakdown of the paint coatings 4. Seawater corrosion in the majority of cases, this will take place in the ballast tanks. Many companies now place sacrificial anodes in the ballast tanks, which considerably reduce the corrosive effect of air and saltwater 5. Under SOLAS Chapter II-1 double side skin spaces must be provided with a compliant protection coating. Fig : Cargo hold construction of a typical bulk carrier



Metal fatigue The weakening of the steel in a structure due to constant flexing, under the repeated cycles of stress may result in structural fatigue failure. The concern about fatigue failure is that it occurs without any apparent forewarning (eg deformation of a structure that results in a crack). Fatigue usually begins at welded joints, notches, discontinuities in structures and areas of high rigidity in particular. However, variations in the size, shape and design of each component and the conditions that the ship operates mean this may not necessarily result in a structural failure. Areas where extra vigilant inspection is recommended include: 1. The brackets at the connection of frames to the upper and lower wing tanks 2. the upper and lower connection of corrugated transverse bulkheads 3. corners of the hatch coamings where they are joined to the main deck. Bulk carriers in particular become progressively weaker due to continuous corrosion. In addition, the repetitive cycles of changing loads and the resulting stresses due to hogging, sagging, panting, pounding and vibration all increase fatigue. High tensile steel (which is stronger than mild steel) is used in all areas likely to experience high levels of stress. It means that scantlings can be reduced but the vessel will still have higher strength and resistance to stresses, eg slamming due to heavy pitching that may cause fatigue on the forward section of the hull. It is recommended that, as soon as any cracks are seen, arrangements are made immediately to repair them. Where possible, a crack arrestor hole should be drilled at each end of the crack before any temporary repair is made. If the extent of the crack is not evident, a detector dye can be used to establish this. As soon as possible, Class should be called for a survey to make a permanent repair because a crack that is overlooked may become a central point for localised stress resulting in structural failure. A crack may also damage protective coatings such as paintwork, creating an `open' area for corrosion. While cracks may not initially be apparent, corrosion in any area should be carefully checked for signs of minor cracks, particularly if there are dents in the structure. Operational Factors Corrosion and fatigue will gradually weaken the hull over time. This can be



increased by variations in loading patterns and particularly heavy density cargoes such as iron ore. Another factor that gradually weakens a ship's structure is the abrasive and corrosive nature of bulk cargoes such as coal, which can cause unintentional damage to cargo hold coatings. Areas such as welded frame joints with tanktop or deck plating are very likely to develop corrosion and subsequently crack if the coatings are damaged. Other factors include: 1. Liquefaction of cargoes, caused by water ingress or moisture in the cargo, can cause cargo shift during the voyage 2. movement of ballast water in partly filled ballast water tanks or holds can cause damage and create corrosion. To avoid this, tanks and holds should be completely filled. Fig: These holds are unlikely to pass a grain survey, as they are heavily pitted with rust scale and embedded with coal staining



Cathodic protection Cathodic protection is a system of preventing corrosion by forcing all surfaces of a structure (e.g. hull) to be cathodes by providing external anodes. It can be achieved by superimposing on the hull an impressed current provided by a remote power source through a small number of inert anodes (impressed current cathodic protection). Also accomplished by fitting aluminium, magnesium or zinc anodes in tanks or underwater portion of a ship, which waste away by galvanic action (sacrificial anode cathodic protection).



Effect of corrosion that could arise due to Structural stress Uneven distribution of cargo



Ships are primarily exposed to atmospheric corrosion, caused by a combination of high moisture and salt-laden sea spray, both of which directly attack the steel through the smallest deficiencies of the pain layer. Ships also suffer from fretting corrosion, caused by the repeated relative surface motion between loaded metal surfaces, typically induced by vibration (caused by machinery) and structural flexing (caused by sea currents and wind). Like all metal structures containing different metals, at the contact points between different metals, galvanic corrosion takes place – by the two metals forming a parasitic galvanic cell with the sea water acting as an electrolyte. This cell’s action causes one of the metals, typically the steel to oxidise. Weak or absent earth connections between a docked ship’s hull and its pier-side power supply causes leakage earth currents to flow out of the ship’s hull (typically through a hull protrusion or sharp edge) into the water, and thereafter to the seabed, which is the harbour power-supply’s earth. The point at which current leaves the ship’s hull is prone to stray current corrosion. The abundance of sea spray causes water to become trapped and accumulate in crevices, whether formed by silt, sand, marine organisms, structural recesses, or sealing material. This forms a stagnant always humid area on the metal, which gives rise to a hidden-from-the-eye corrosion patch, which gradually erodes the metal and allows water to ingress inner structures. This is known as crevice corrosion. Microbial corrosion is caused by certain types of marine bacteria, which stick to the hull or accumulate in crevices. Their biological activity modifies local chemistry by acid production, thus accelerating corrosion. Internal corrosion takes place in tanks, piping and pumping equipment in oil tankers.     CORROSION VICIOUS CIRCLES Structural weakening – Since stress-bearing components of ships are made of steel, any serious corrosion causes structural weakness, compromising safety – In stormy seas, ships’ hulls are subjected to much torsional stress, and normal-duty payload (e.g. ship cargo) also stresses the structure. A weakened structure will flex more, increasing fretting corrosion, and in turn further weakening the structure, as well as allowing the ingress of water and dirt through weakened seals or through welded/rivetted joints becoming porous, as explained below. And of course, a sufficiently weakened structure can catastrophically fail. Ingress of water and dirt causing more corrosion – Corrosion around openings, often caused by salt water trapped under rubber seals, weakens the effectiveness of these seals and allows water and dirt to enter enclosed areas. Those enclosed areas may have an intricate internal structure with many places where water and dirt can accumulate – usually invisible from the outside. The accumulated dirt forms a



sponge which retains any water ingress, forming stagnant pools of salty water, causing further corrosion. Ingress of water causing electrical faults – The weakening of seals mentioned above can also cause water to enter electrical connection boxes and equipment, causing short circuits and the corrosion of electrical connections. Ingress of water causing flooding – Corrosion-weakened seals may cause ingress of enough water to destabilise the ship. Corrosion-weakened rivetted or welded joints (which may be below the water line) may become porous, allowing massive flooding.